Frequency modulation discriminator circuit



Nov.` 28, 1944. M, cs. cJRoslaYV 2,363,652

` FREQUNCY MODULATION DISCRIMINATOR CIRCUIT Filed April 2s, -1943 2sheets-sheet 1 .O FF- TUNE "ATTORNEY Nov.4 28, 1944.' I M. G. cRosBY 12,363,652

FREQUENCY MODULATION'DISCRIMINATCR CIRCUIT l 'S C Fc -FfEgz/s/vcr I IH7b A j L 3) M.. Mmmm-.- '576ML r1 j l 007.007 5 9' al; ///`=E I INVENTORATToRNEY Patented Nov. 28, 1944 FREQUENCY MoDULA'rToN DISCRIMINA- 'ronCIRCUIT Murray G. Crosby, Riverhead. N. Y., assignor to .RadioCorporation oi' America, a corporation of Delaware .I

I Application April 29, 1943, Serial No. 484,973

j 9 claims. (ci. 25o-27) MyV present invention relates generally tofrequency discriminator circuits, and more particularly to novel formsof discriminator circuits for frequency modulated (FM) carrier waveenergy.

An object of my invention is to provide an FM detector circuit wherein apair of opDOSed rectiers is fed by a discriminator which consists of aresonant circuit path for feeding both rectiflers in parallel withretarded FM 'signal energy, and additional paths for feeding therectiiiers with unretarded FM signal energy, but in opposite polarity.

Another object of my invention is to provide l a discriminator for FMsignal energy wherein there is provided a parallel resonant circuit pathfunctioning to feed retarded FM signal energy to a detector forfrequencies oif the predetermined frequency of the parallel resonantcircuit, there Vbeing provided a second path to the detector circuit forfeeding to the latter unretarded FM signal energy.

The novel features which I believe to be characteristiocf myfinventionare set forth with particularity in the appended claims; the inventionitself, however. as to both its organization and method of operationwill best be understood by reference to the following description, takenin connection with the drawings, in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings: Fig. 1 shows ,one form of a discriminator-recti fiercircuit embodying the invention,

Fig. 1a shows a modification of the circuit arf rangement of Fig. 1,Y

source of the signal energy is not disclosed, because it may be ofanywell known form. For example, where the invention is used in asuperheterodyne receiver of FM signal energy it will.

be clear that the transformer 3 maybe an intermediate frequency (I. FJtransformer which is fthe series resonant circuit S. This series reso-.or low deviation ranget fed fromV the'plate 'circuit of a prior stageof I. F. amplification, or from the plate circuit of an amplitudelimiter tube. In any event the primary circuit l of transformer 3 isxedly tuned to theoperatlng I. F. value of the receiver. The FM signalenergy itself may either have a high A ain, the FM signal energyreceived at the signa collector device of the receiver may be in themegacycle (Mc.) range,

and the I. F. value itself may be in the megacycle or kilocycle bands.

'I'he secondary winding "I of transformer 3 has its upper end connectedto the anodes of rectiiiers I and 2 through s. path which includes nantcircuit S is tuned to the mean, or center, frequency of the applied FMsignal energy. This frequency will, of course, be the Vfrequency towhich the primary and 'secondary-circuits of 4 transformer 3 are tuned.The series resonant cir- Figs. 2a and 2b show graphically the vector?,

relations between the retarded and unretarded signal energies of Figs. 1and 1a for the "in-tune condition as well as for the off-tune"vcondition, A

Fig. 3 shows a modified form of the invention, Fig. 3ashows thefrequency-amplitude char-f acteristic lof the parallel resonant circuitof thev `discriminator in Fig. 3,

Fig. 3b vectoriallyY illustrates the olf-tune condition of thediscriminator in Fig. 3, y

Fig. 4 shows a circuit embodying a'. modiiica-A tion' of thediscriminator of Fig.13.

Referring now to the accompanying drawings.- wherein like referencecharacters in the different figures designate similar-elements, in Fig.1 the opposed rectifiers I and .2, specifically disclosed as of thediode type, function as a means for rectifying 'signal energy appliedthereto. The

cuit specifically consists of condenser 8 and inductance 9. The outputterminal of the series resonant circuit S is connected to the respectiveanodes of diodes I and 2 through resistors I0 and II respectively. 'I'heopposite ends of winding I are respectively connected to the anodesA ofrectiersJ and 2 through non-selective paths s consisting of respectivecondensers 5 and 6. The

" condenser 5 shunts the series resonant circuit'S and resistor l0. Thecondenser 6 is connected directly from the lower end of winding 1 to theanode of rectifier 2. The` midpoint of winding 1 is effectively'grounded for radio frequencies by virtue of its connection to lgroundthrough a path which includes the load resistor I2.

This load resistor I2 is arranged in series with load resistor I3, andthe cathode end of resistor I2 is grounded. Each of resistors I3 and I2is shunted by a respective carrierfrequency bypass condenser. ResistorIl is a direc current return resistor connected between the node ofrectier I andthe junction of resistors I3 and I2.4

Resistor I5 functions as a direct current return resistor, and isconnected between the anode ofrectier 2 and the aforementioned junctionpoint. It will be understood that from the cathf odeend of resistor I3there is derived the difage of each of rectiiiers I and 2,. Thisdiiferenferential resultant of the rectiiied output volttial resultantvoltage may be the modulation signal voltage, such as audio frequencyvoltage, which is utilized in subsequent audio frequency amplifiers.There may, also, be taken off from the cathode end of resistor I3automatic frequency control (AFC) voltage for correcting the frequencyof the local oscillator of the super-` heterodyne receiver so as tocompensate forany relatively slow shifts in the mean frequency of the FMsignal energy applied to the input transformer 3.

Considering the functioning of the network of Fig. 1 it is pointed outthat for the in-tune condition, depicted in Fig. 2a, Vthe Voltage whichis -fed to each of the rectiiers through the series resonant path S maybe y represented yby the Vector Es. In the in-tune condition of theapplied signal energy the mean frequency of such applied energy is, ofcourse, equal to the resonant frequency of seriesresonant circuit S. yThe circuit S, as is well known, retards or shifts the phase of Ithesignal voltage applied thereto to an increasing degree for frequenciesdeparting from the mean or carrier frequency value. Hence,

l there will be no phase shift of the signal voltage applied to S at theinstant when its mean frequency is equal to that of circuit S. Since theresonant circuitI S feeds the rectiilers through resistive elements Iand II, it follows that at the instant when the applied signal energy isat the mean frequency there will be no phase shift provided in thesignal energy developed Es in Fig. 2a. The signal voltage which passesthrough condensers 5 and 6 respectively has not applied to therespective rectifiers I and 2 are now substantially as represented inFig. 2b. One of these resultant vectors exceeds the other in magnitude,and the differential voltage derived from the cathode end of resistor I3has some value.

The value of the differential voltage is dependent at the outputterminal of S which is fed in like j polarity to the rectiflers. Thiswould' alsobe true at other instantaneous frequencies of the signalenergy.

However, the signal Voltage fed through condensers E and 6 is derivedfrom the opposite ends of winding 1. Since the midpoint of winding 1 iseffectively grounded for radio frequency energy,

it follows 4thatthe voltages fed through condensers 5 and 6 arerespectively of l different polarity. Furthermore, these voltages have aphase shift'of 90 degrees with respect to the retarded voltage fedthrough the series resonant circuit S. The condensers 5 and 6 do nothave the selective phase shift characteristic of circuit S. 'I'heyaffect the signal energy in a non-selective manner at all instantaneousfrequencies.

Fig. 2a shows the vector representations of the voltages Ee and E5 whichare fed through con'- vdensers 6 and 5 respectively. It will, therefore,be

plied signal energy is different from the resonant frequency of seriesresonant circuit S, there is produced aphase shift in the voltage fedtherethrough. As previously explained this phase shift is increasinglygreater in proportion to the departure of the instantaneous signalfrequency from the vmean frequency thereof. The vector E; in Fig. 2brepresents the voltage now'fed through the series resonant circuit, andit will be seen that some phase shift has been provided in this vectorrelative to the angular' 110511.71013 Qf upon the magnitude of frequencydifference between the instantaneous frequency of the applied signalenergy and the predetermined frequency of series resonant circuit S. Inthe case where there occurs a relatively slow departure ofthe signalenergy mean frequency from the aforesaid predetermined frequency ofcircuitlS, the differential voltage is used for AFC voltage. The

relatively fast frequency deviations of the signaly energy from saidpredetermined frequency show up as modulation variations of saiddifferential voltage.

In Fig. 1a. there is shown a modification wherein the series resonantcircuit 8-9 is fed from a single resonant circuit 3 located in the platecircuit of the prior amplifier tube 4. The coil of tuned 4circuit 3' iscenter-tapped by the +B voltage line which feeds the plate of tube 4.The +B line is bypassed to ground for signal frequencies, and thereforethe center tapis at ground potential for signal frequencies. In this waythere is provided the opposite polarity voltages for the paths 5 and 6.The circuit is otherwise similar in action to that of Fig. 1. Theadvantage of this circuit is that the two circuits 4 and I of Fig. 1have been replaced by a single input circuit.

It is not necessary to employ a series resonant circuit in the retardingor common selective phase shift path to the opposed rectifiers. In Fig.3 I have shown a parallel resonant circuit T which specifically consistsof an inductance 9' and a shunt condenser 8. The parallel resonantcircuit Tis tuned, as was the series resonant circuit S, to the desiredor predetermined mean fre- 'quency of applied signal energy. That is,the

circuit T is tuned to the same frequency as the input transformer 3. Inthe case of a superheterodyne receiver the transformer circuits and Twill be tuned to the operating I. F. value. There is a difference inoperation in this modification by virtue of the nature of the frequency-V amplitude characteristic of circuit T.

resonant circuit T is'zero. For frequencies less than, or greater than,the resonant frequency ol circuit T, -the amplitude of the transmittedcurrent increases in magnitude in a positive or negative senserespectively. The positive increase in current is inductive in nature,whereas the negative increase in current is capacitive in nature.

Those skilled in the art are fully aware of this` exist for theoff-tunecondition of the circuit of Fig. 3. The vectors Es and E5 are similar tothose Shown in Fig. 2b. That is, the FM signal energy aaeacta'transmitted throughcondensers V and "6 are shifted 90 degrees in phase,and are appliedto` the rectifiers 4I and 2 in opposite polarity. At thein-tune condition, that is when the mean-fre quency of the appliedsignal energy is exactly equal to the frequency Fc of tuned circuit T,the vector Et representing-the voltage transmitted through circuit T iszero. Therefore, there will be applied to the opposed rectiiiers I and 2voltages E5 and Es respectively, and the differential outputvoltage willbe zero. However, for the `olf-tune condition the vector Et will have alength which depends on the frequency difference betweentheinstantaneous frequency of the applied signal energy and thefrequency Fc. The vector. Erwill be in aiding phase with eitherofvectors Es or Es depending upon the direction in which the instantaneousfrequency of applied signal energy differs from Fc. This follows fromthe amplitude-frequency characteristic of Fig.'

3a. The signal current through circuit T will be in phase with thecurrent through condenser 5 or condenser 6 depending upon the 'directionof frequency deviation of the signal energy relative to Fc. Accordingly,the vector Et will be added on either. of vectors Es or Ea, and in thisway-one of the rectifiers will have signal voltage` of larger magnitude.applied thereto than the other rectifier. It will, therefore, be notedthat for thearrangement shown in Fig. i the resultant vectors areproduced by virtue of an angular dierence between the retarded'andunretarded voltages, whereas-for the arrangement shown in Fig. 3 the'resultant vectors arise by virtue of a Vsimple additive, or aiding,phase relation between one of the unretarded voltages Ee or E5 and theretarded voltage Et. V

In Fig. 4 I have shown a further modification which is essentiallybasedr on thel arrangement shown in Fig. 3. In the case of Fig. 4 the 90degree phase shift effected by the tuned transformer 3 is utilized toproperly phase' the combination of voltages fed to each diode.transformer is tuned for band pas's action, and may be properly dampedby primary and secondary resistors 20 and 2|, as shown, or the dampingmay consist of the losses in the primary The with voltages induced 'in'in the manner of Fig. 3b.

While I have indicated and described several systems for carrying myinvention into eect, it will be apparent to one skilled in the art thatmy invention is by-no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set of opposed rectiiiershaving a differential'output circuit, a discriminator circuit feedingthe op posed rectiers, a source of frequency-variable signal energycoupled to said discriminator, the

' improvement inthe discriminator comprising a by to apply retardedsignal voltage to said rectilfiers in opposite polarity.

2. In combination with a source of frequency variable signal energy anda pair of opposed rectiers having a differential output circuit, adiscriminator circuit comprising a retarding path consisting of aparallel resonant circuit tuned to the mean frequency ofappliedfrequency-variable signal energy. means 'for applying the output of saidparallel `resonant circuit in parallel to said rectiiiers, a second pathfeeding the signal energy to one of the rectiiiers with a relative phaseshift of 90 degrees, and a third path feedingthe signal energy to thesecond rectifier in opposite polarity to the signal energy fed throughthe second pathA but'with a similar quadratureV phase shift.

and secondary coils, 'I'he midpoint of winding 1 is connected to thejunction of the load resistors I3 and I2'through a parallel resonantcircuit T'.

The midpoint of winding 'l is additionally con` nected to the highpotential side of resonant circuit I through a path consistingofrblocking condenser 5 in `series with the parallel resonant circuit T.The opposite side of resonantl circuit 4 is connected to the midpoint of-winding 1 through a. path which consistsof blocking condenser 6 inseries with an adjustable resistor R.. 'I'he midpoint of the primarywinding of transformer 3 is connected to the +B line of the directcurrent voltage supply source for the plate of the immediately precedingamplifier tube. Here. again, the +B line is bypassed to ground forsignal frequencies.

In Fig. 4 thev voltages Es and Es are. fed to diodes l and 2 throughmid-tapped winding 1.

En is fed through T which has its resistance balirnpedance across whichvoltage of T and R appear"The highside of 'IT' is connected'to themid-tap of 1 so that the voltage across T is fed inphase to the opposeddiodes, and combines 3. In a system of the type 'comprising .a pair ofopposed rectiflers having a differential output circuit, a discriminatorcircuit feeding the opposed rectiers, lafsource of frequency-variable 'Ysignal energy coupled to said discriminator, the improvement in thediscriminator comprising a signal energy retarding path consisting of aseries i resonant circuit" tuned to the mean frequency of applied signalenergy, aperiodic means connecting said series resonant vcircuit inparallel to said rectiflers, and a pair of non-retarding pathsconnecting said source to respective rectiflers thereby to applyretarded signal voltage to saidrectifiers in opposite polarity, and saidaperiodic means functioning to isolate said retarding -path from thenon-retarding paths.

4. In combination with `a source of frequencyvariable signal energy anda pair of opposed rectiers having a differential output circuit, adiscriminator' circuit comprising a retarding path consisting of aparallel resonant circuittuned to the mean frequency of appliedfrequency-variable signal energy, meansV for applying the output of saidparallel resonant circuit in parallel to said rectiers, a second pathfeeding the signal energy to one of the rectiflers with a relative phaseshift of degrees, and a third path feeding the signal energy to thesecond rectifier in opposite polarity to the vsignal energy fed throughthe-second path but with a similar quadrature phase shift, and

isolation` resistors'being inserted between said parallel resonantcircuit and third'paths.- y

5. In a frequencyidlscriminator network inand each of said second-cluding a pair of opposed rectiilers, a signal retarding path'correlatingl of a tuned circuit composed of separatereactances ofopposite sign,

means for applying retarded signal voltage to the rectifiers in likepolarity. separate non-selective paths for feeding unretarded signal`voltages to the rectiilers in opposite polarity, and said applyingmeans being constructed and arranged to isolate the signal retardingpath from the separate paths.

6. In a frequency modulation detector, a pair of opposed rectifiers, adiscriminator circuit for the rectiers comprising a resonant circuitsignal retard path, isolation resistors connecting said resonant circuitin parallel to the rectiilers, and lnon-selective means for feeding thelatter with unretarded signal voltages of opposite polarity.

'7. In a system of the type comprising a. pjur of opposed rectifiershaving a diierential output circuit, a discriminator crcuit feeding.theopposed rectiilers, a source of frequency-variable signal energy coupledto said discriminator, the

improvement in the discriminator comprising a signal energy retardingpath consisting of a series resonant circuit tuned to the meanfrequency'of applied signal energy, aperiodic means connecting saidseries resonant circuit in parallel to said rectier's, and a pair ofnon-retarding paths connecting said source to'respective rectiierstherebyto apply retarded signal voltage to said recti- 8. In combinationwitha pair oi opposed diode rectiers having a differential outputcircuit, a

'diode rectiiiers having a dinerential output circuit, a discriminatorcircuit comprising a retardy ing path consisting of a parallel resonantcircuit,

tuned to the mean frequency of applied signal energy, isolation resistormeans `for applying the output of said parallel resonant circuit inparallel to said rectiiiers, a capacitative path feeding the signalenergy to one of the rectiers with a relative phase shift of 90 degrees,a second capacitative path feeding the signal energy to the secondrectifier in opposite polarity to thesignal energy fed through the firstcapacitative path but with a similar quadrature phase sluit. 1

- MURRAY G. CROSBY.

